A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Commonly, the substrate and/or patterning device (e.g. the reticle) are supported and positioned by a respective moveable stage system including a displacement device to move the stage system relative to a frame, e.g. a base frame or another kind of reference structure. Such displacement devices include a first part on the frame and a second part on the stage system which are displaceable relative to one another in a first direction and a second direction perpendicular to the first direction.
The first part includes a carrier which extends substantially parallel to the first and second direction. On the carrier a system of magnets is fastened in accordance with a pattern whereby magnets of a first kind with a magnetization direction perpendicular to the carrier and directed towards the second part and magnets of a second kind with a magnetization direction perpendicular to the carrier and directed away from the second part are arranged in accordance with a pattern of rows and columns perpendicular thereto and enclosing an angle of approximately 45° with one of the first or second direction, such that the magnets of the first kind and the magnets of the second kind are arranged in each row and in each column alternately. In this application, magnets of the first kind and magnets of the second kind may alternatively be referred to as first and second magnets respectively.
An example of such a carrier is shown in FIG. 2A, wherein the magnets of the first kind are denoted N, the magnets of the second kind are denoted Z, the first direction is denoted X, and the second direction is denoted Y. Hence, the magnetization directions of both the magnets of the first and second kind (N,Z) are parallel to a third direction, wherein the third direction is perpendicular to both the first and second direction.
The second part is provided with an electric coil system having a set of coil block units, each coil block unit having current conductors which are situated in a magnetic field of the system of magnets and are being fed by a multi-phase system.
Known displacement devices normally have a set of four coil block units as shown in FIG. 2B, wherein two coil block units are of a first kind in which current conductors are oriented in the second direction to apply a force between the first and second part in the first direction based on Lorentz law. The other two coil block units are of a second kind in which current conductors are oriented in the first direction to apply a force between the first and second part in the second direction based on Lorentz law. Usually the coil block units are also able to provide a force between the first and second part in the third direction, e.g. to levitate the second part relative to the first part. In FIG. 2B, a coil block unit is denoted CB, the current conductors are denoted CC, and the first and second direction are denoted X and Y respectively.
Currents through the current conductors of a coil block will generate forces depending amongst others on the orientation of the local magnetic field. However, the currents can be driven such that the sum of the forces is applied in the respective first or second direction. If also the current conductors are used to levitate the second part, a levitation force can be applied independently from the force in the first or second direction, thereby allowing full control of the movement of the stage system in all applicable directions.
Each coil block unit is thus a force generating element having a maximum force that it can generate, the maximum force being determined amongst others by the magnetic field strength, amount and location of current conductors, and a maximum allowable current.
When the coil block units are driven to generate their maximum forces in respectively the first, second direction, and possibly third direction, there will be a line or point in which the moment generated by the forces is zero. This line or point will be referred to as the center of force COF. In FIG. 2B, the center of force COF is indicated for the situation that all main coil block units are able to generate the same maximum force.
Preferably, the center of force fully coincides with the center of gravity of the second part including the parts supported by the second part as a mismatch between center of force and center of gravity will require a higher maximum force at least for one of the coil block units to achieve the same level of performance and will require compensation for the created moment. However, due to practical reasons, the center of force normally only coincides (approximately) in the first and second direction with the center of gravity and not in the third direction as aligning the coil block units with the center of gravity in the third direction would require a relatively large distance between the magnetic field generating first part and the current conductors of the second part which is undesired from a performance point of view.
Due to e.g. design modifications, the center of gravity may be shifted when comparing different stage systems. To comply with the desired requirement that the center of force at least partially coincides with the center of gravity, a design modification changing the center of gravity also requires a change of the displacement system. Current displacement systems can change their center of force, but at the cost of dramatic changes in design, space, etc. as will be explained with reference to FIG. 2B.
FIG. 2B discloses a conventional second part of a displacement device including four coil block units, two of the first kind and two of the second kind, the coil block units being arranged in a diagonal configuration, i.e. symmetrically arranged around one point. Assuming the coil block units are able to generate the same amount of force, the center of force is located at this symmetry point. When the center of gravity does not coincide with the center of force, the center of force is preferably shifted towards the center of gravity. However, shifting of the center of force requires undesired changes. For instance, shifting all coil block units will not only shift the center of gravity as well, but it also requires a design modification of the coil block unit carrying frame to which the coil block units are mounted. Shifting only a few coil block units will result in a change in size and again a modification of the coil block unit carrying frame.
Another possibility would be to modify the maximum generable force of a coil block unit, but this would require a change of the coil block unit itself, e.g. in terms of size and/or cooling which is also undesirable.
In prior art displacement devices, the magnetic fields generated by the magnets on the first part and the coil block units on the second part may have a significant influence (referred to as crosstalk) on additional positioning devices arranged nearby, e.g. a positioning device provided between the second part and a third part to position the third part relative to the second part. The crosstalk may manifest itself as disturbance forces for the additional positioning devices which are thus more difficult to control and may lead to inaccurate positioning of other components such as the third part.